Projection objective and waveguide display device

ABSTRACT

The invention concerns a projection objective and a waveguide display. The objective is adapted to project an image from a first plane to a second plane and comprises in order from the second plane a first optical element group having a positive effective focal length, a second optical element group placed between the first plane and the first optical element group and having a negative effective focal length, and a third optical element group placed between the first plane and the second optical element group and having a positive effective focal length. Counting from the second plane, the first refractive surface of the second optical element group is concave towards the second plane and the second refractive surface of the third optical element group is convex towards the first plane. The objective suits well for projecting images to diffractive optical displays.

FIELD OF THE INVENTION

The invention relates to projection objectives. In particular, theinvention relates to a telecentric projection objective for augmentedreality (AR) applications and the like.

BACKGROUND OF THE INVENTION

Recent interest in augmented reality applications and augmented realitysystems has increased the number of different optical methods inachieving the overlap between projected image and the real background.

A specific type of display used in AR systems is a waveguide typedisplay, where light is coupled in to a piece of glass or polymer in anin-coupling region and the light propagates therein within the totalinternal reflection limit angle until it is coupled out from the lightguide at an out-coupling region. Ray splitting at the out-couplingregion can effectively expand the eye box of the system at the cost ofbrightness.

Waveguide type displays can principally be split to two differentgroups. In the first group the projection objective exit pupil isexpanded along one dimension only in the waveguide.

In the second group the projector exit pupil is expanded along twodimensions in the waveguide. The present invention relates in particularto the second group in which the requirements for the incoming light arestricter and which is therefore optically more challenging. For example,the exit pupil of the projector must be of certain diameter and close tothe in-coupling grating of the waveguide, the projected beam of lightmust be highly collimated and telecentric.

Exit pupil expansion in the waveguide can be achieved in several ways,diffractive elements and partially reflective surfaces being the mostcommon.

In practice, it is desirable in augmented reality applications that thetotal volume of the projection system is as small as possible. Ingeneral, many small-volume objective systems are known from the field ofmobile phone camera technology. For example, U.S. Pat. Nos. 7,453,654,7,502,181, 7,826,151, 7,965,455, 8,035,723 and 8,953,262 describe fixedfocal length mobile phone camera objectives where the aperture stop islocated between the object and second lens element, typically residingjust in front of the first lens element to minimize the objective size.The compactness of these objectives requires the second lens element tobe a negative lens element. In addition, telecentricity of theseobjectives is typically not very high. The mobile phone camera solutionsare not as such applicable to the projection part of waveguide typedisplays and are in particular not capable of being used for expandingprojector exit pupil along two dimensions in combination withdiffractive gratings.

Thus, there is a need for improved projector objectives for waveguidedisplays.

SUMMARY OF THE INVENTION

It is an aim of the invention to solve at least some of theabovementioned problems and to provide a novel projection objective thatcan be used to couple light in to a waveguide which effectively expandsthe projector exit pupil along two dimensions.

A particular aim is to provide a projection objective that produces acollimated and telecentric enough beam for waveguide-based displayshaving a diffractive in-coupling grating. A further aim is to provide anobjective, whose exit pupil can be brought close enough to the grating.

An aim is also to provide a projection objective, whose optics can befitted in a small space.

The invention is based on providing a projection objective with apositive-negative-positive lens grouping with specific refractivesurface configuration that allows for high telecentricity of theobjective and collimation of light.

In more detail, the present projection objective is adapted to projectan image from a first plane to a second plane, the objective comprisingin order from the second plane a first optical element group having apositive effective focal length, a second optical element group placedbetween the first plane and the first optical element group and having anegative effective focal length, and a third optical element groupplaced between the first plane and the second optical element group andhaving a positive effective focal length. Each group comprises at leasttwo refractive surfaces, typically in the form of lens surfaces.Counting refractive surfaces of the groups from the direction of thesecond plane, the first refractive surface of the second optical elementgroup is concave towards the second plane and the second refractivesurface of the third optical element group is convex towards the firstplane.

It should be noted that although the optical element groups and surfacesare herein listed from the direction of the second plane towards thefirst plane, in projection applications herein discussed the lighttravels from the first plane towards the second plane.

The present waveguide display device comprises an optical waveguidehaving an in-coupling grating arranged thereto so as to diffract lighthitting the in-coupling grating to the waveguide, and a projectionobjective of the present kind arranged to project an image from an imagesource to the in-coupling grating.

The in-coupling grating can be arranged at the second plane, which canalso be the plane of the aperture stop of the projection objective. Theprojector is adapted to present the image to be projected on the firstplane of the objective.

In particular, the projection objective has a projection exit pupildetermined by its aperture stop (that is, there are no further imagingoptics behind the aperture stop). The waveguide of the waveguide displaymay comprise means, in particular diffractive grating means, forexpanding the projection exit pupil along two dimensions.

In particular, the invention is characterized by what is stated in theindependent claims.

The invention offers significant benefits.

First, by means of the invention, it is possible to achieve a verycompact projection objective capable of producing a collimated beam ofprojected light such that the exit pupil of the projection objective isclose to the in-coupling grating of the waveguide.

In addition, the present optical configuration allows for manufacturinga telecentric or nearly projection objective, meaning that the maximumfield of view chief ray arrives to the display nearly perpendicular.This has a remarkable positive effect on light engine efficiency.

The dependent claims are directed to selected embodiments of theinvention.

Next, embodiments of the invention and advantages thereof are discussedin more details with reference to the attached drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A-1E show in cross-sectional views different embodiments ofoptics of the present objective.

FIG. 2 illustrates the optical setup between the third optical elementgroup and the first plane.

DESCRIPTION OF EMBODIMENTS

In the following discussion, various embodiments of the invention areintroduced that allow keeping the objective size small, incorporatingsufficient back focal length to the design and enforcing thetelecentricity condition on the edge field rays.

Relative referrals, such as “before”, “after”, “first” and “last”,unless otherwise apparent, are herein made with reference to the traveldirection of light from the first plane (projector) towards the secondplane (waveguide display). Optical element groups, optical elementstherein and refractive surfaces are, however, numbered in the oppositeorder from the second plane towards the first plane.

In embodiments of the present invention, the objective is divided intothree groups, which may contain one or more optical elements, inparticular lenses. The elements may or may not be aspherical. In FIGS.1A 1E, the three groups are denoted with G1, G2 and G3, starting fromthe second plane. The second plane is denoted with reference numerals10A 10E and the first plane with 20A 20E, respectively. The projectordisplay (the object to be projected), such as an liquid crystal onsilicon (LCOS) or digital light processing (DLP) display, is positionedat the first plane 20A 20E. A protective glass of the projector displayis denoted with numerals 19A 19E. The in-coupling grating of thewaveguide display is typically positioned at the second plane 10A 10E.From the in-coupling grating, light is coupled to a waveguide, in whichit propagates via total internal reflections.

Generally speaking, the aperture stop of the objective is located at thesecond plane or between the second plane and the second optical elementgroup G2, typically at the second plane or between the second plane andthe first optical element group G1. In the illustrated examples, theplane of the aperture stop coincides with the second plane, where thein-coupling grating is positioned. In particular, the aperture stopresides after the lens elements of the first group G1, since this allowsthe use of a simple the in-coupling grating design.

The first element group G1, like the second and third groups G2, G3, maycomprise a single lens element or two more lens elements togetherproviding the required effective focal length for the group. Two or moreelements may be attached to each other without air gap between thelenses in order to form a doublet or triplet, for example.

In the embodiments of FIGS. 1A, 1C and 1D, there is provided in thefirst group G1 a biconvex lens 11A, 11C, 11D. In the embodiment of FIG.1B, the first group comprises a planoconvex lens 11B. In alternativeembodiments, illustrated in FIG. 1E, instead of a biconvex orplanoconvex lens, the first group G1, comprises or consists of apositive meniscus lens or lens doublet 11E.

There may be provided additional elements (not shown) in the first groupG1, such as a prism. The additional elements may be on either or bothsides of the lens or lens group 11A-11E, for example between theaperture stop at the second plane 10A 10E and the lens. In theembodiment of FIG. 1E, a prism schematically illustrated by a thickglass plate 12E, in positioned between the lens 11E and the second groupG2. In some embodiments, there are no such additional elements, wherebythe lens or lens group 11A 11E is the only element of group G1, and alsothe last element of the projection objective before the waveguide at theaperture stop.

The second group G2 contains negative refractive power lens element 14A14E or a plurality of element together providing negative effectivefocal length. This group provides for the fastest expansion of the lightrays, which together with the third positive group G3 satisfies thetelecentricity requirement by intercepting the field chief raysapproximately at the required image height. Thus, the negative lensspreads the beam of light within a short distance such that the chiefray is guided far from the optical axis and the positive group thenrefracts the beam so that it hits the in-coupling grating of the displayelement substantially at normal angle.

The first refractive surface of the second group G2 having a negativerefractive power provides for greatest telecentric expansion in theshortest distance possible. It is also preferred that the second groupG2 has an effective focal length that is shorter than the total focallength of the projection objective. Thus, the second optical elementgroup G2 significantly contributes to the compactness of the projectionobjective. It should be noted that in the case of e.g. mobile phonecamera lenses, there is no strict telecentricity requirement, wherebyalso the negative optical elements thereof have different kind ofdesign, typically with the image side surface, instead of the objectside surface, of the lens element having the greatest negativerefractive power.

In the embodiments of FIG. 1A and FIG. 1B, the second group G2 consistsof a biconcave lens 14A, 14B. In the embodiments of FIGS. 1C and 1D, thelens in this group G2 is a negative meniscus lens 14C, 14D. In theembodiment of FIG. 1E, the second group G2 consists of a doublet lens,herein more specifically a negative meniscus lens 14E and biconcave lens15E doublet.

The third group G3 contains the remainder of necessary lens elements,together having a positive effective focal length, between the secondgroup and a following prism or beam splitter assembly 18A 18E. Positivefocal length allows for focusing the light rays once the divergence hasbeen increased by the second group G2.

In some embodiments, the third group G3 comprises two or more lenselements, the first of which has two refractive surfaces, of which thesecond is convex towards the first plane and at least one of the otherlens elements is a positive lens element, for example a planoconvex lenselement or an aspherical lens element.

In the embodiments of FIGS. 1A and 1E, the third group G3 consists ofone positive biconvex element 16A only. In the embodiment of FIG. 1B,there are provided two positive elements, i.e., a positive meniscus lens15B and a biconvex lens 16B. In the embodiment of FIGS. 1C and 1D, thereare provided a positive meniscus lens 15C; 15D followed by a lens pair16C, 17C; 16D, 17D. This lens pair can be designed such that the firstlens 16C; 16D has a higher refractive power. In particular, the thirdoptical element group may comprise two or more lens elements, the firstof which 16C; 16D has a refractive surface that is convex towards thefirst plane 20C, 20D. The second lens element 16C; 17D of the lens pairmay have refractive surface that is convex towards the second plane 10C,10D. One or both of lenses 16C, 17C; 16D, 17D can be an aspherical lens.This allows for correcting the wavefront for improving image quality(resolving power of the objective).

The arrangements of FIGS. 1C and 1D differ in that the size of theprojection display is different. FIG. 1C represents a 50 degree systemand FIG. 1D a 60 degree system.

Before the third optical element group G3, there may be provided, aselement 18A 18E, a prism that expands the optical path of rays. A prismcan be used in particular in the case of a DLP display. Due to theprism, the first plane 20A 20E can be at a right angle with respect tothe optical axis of the element groups G1 G3. The prism is preferablythe first element of the objective, whereby the distance between theprism and the projector display can be kept as short as possible.

In some embodiments, in particular those with an LCOS display, theelement 18A 18E is a polarizing beam splitter, such as a beam splitterplate or cube.

The relatively large prism, beam splitter cube or a beam splitter mirror18A 18E of the projection objective, typically covering the whole imagearea, is accommodated within the back focal length of the projectionobjective. This increases the back focal length requirement of theprojection objective significantly from e.g. known mobile phone camerasystems, which are optically not compatible with such arrangement.

In a preferred embodiment of the invention the following conditions aresatisfied:

-   -   The aperture stop is located after the second group G2,        typically after the first group G1, in particular at the second        plane, where the in-coupling grating of the waveguide display is        positioned.    -   The first group G1 has a positive effective focal length with        the first refractive surface in this group being convex or        concave towards the second plane (that is, towards the        in-coupling grating of the waveguide display).    -   The second group G2 has a negative effective focal length with        the first refractive surface in this group being concave towards        the second plane.    -   The third group G3 has a positive effective focal length with        the second refractive surface in this group being convex towards        the first plane (that is, towards the projector display).

In further embodiments of the invention one or more of the followingconditions are satisfied:

-   -   The effective focal length f₂ of the second group G2 and the        total effective focal length f of the objective satisfy:        |f₂/f|<0.7 This allows for increasing the telecentricity of the        objective and simultaneously keeping the objective small.    -   The effective focal lengths of the second group G2 and the third        group G3 satisfy: 0.3<|f₂/f₃|<1.5    -   This allows for focusing of the diverging beam coming from group        G2 by group G3, while maintaining high telecentricity and useful        size of the objective.    -   The back focal length (BFL), herein defined as the distance        between the third group G3 and the first plane, and the total        effective focal length f of the objective satisfy 0.4<BFL/f<1.5    -   This ensures that the one can fit typically necessary additional        elements, such as a beam splitter cube or prism, within the back        focal length range. This increases the versatility of the        objective to be used for example with liquid crystal on silicon        (LCOS) projector displays, digital light processing (DLP)        projector displays and organic LED (OLED) projector displays.    -   Image diagonal radius R_(img) and edge field marginal ray height        exiting from the third group M_(rhG3) satisfy:        R_(img)−BFL*0.44<M_(rhG3)<R_(img)+BFL*0.44    -   This restricts the half opening angle of the first lens of the        system to a range that is usable with DLP displays, for example,        and allows for high efficiency and contrast. FIG. 2 illustrates        the variables used herein (without an optional prism shown).

In some specific embodiments, the two last conditions are simultaneouslysatisfied. This ensures a sufficiently small but telecentric objectiveand a sufficiently collimated beam. In a further embodiment, all fourconditions are simultaneously satisfied.

The present projection objective is particularly suitable to be used inpersonal microdisplays, such as near-to-eye displays (NEDs) or otherhead mounted displays (HMDs). In particular, the objective suits foraugmented reality (AR) NEDs or HMDs, in which diffractive waveguides areused for image formation and the projector and its optics must be fittedin a very small space. In some embodiments, the present objective isassembled in an eye-glass format display device, in particular a templethereof, and optically coupled with an image formation device and an thewaveguide of the display. Generally, in AR applications, the waveguidecomprises a transparent light guide comprising, in addition to thein-coupling grating, an out-coupling grating and allowing the user tosimultaneously see through the display and view the projected image.

CITATIONS LIST Patent Literature

-   U.S. Pat. No. 7,453,654-   U.S. Pat. No. 7,502,181-   U.S. Pat. No. 7,826,151-   U.S. Pat. No. 7,965,455-   U.S. Pat. No. 8,035,723-   U.S. Pat. No. 8,953,262

The invention claimed is:
 1. A projection objective for projecting animage from a first plane to an in-coupling grating of a waveguideelement on a second plane, the objective comprising in order from thesecond plane: a first optical element group having a positive effectivefocal length, a second optical element group placed between the firstplane and the first optical element group and having a negativeeffective focal length, and a third optical element group placed betweenthe first plane and the second optical element group and having apositive effective focal length, wherein said optical element groupshave refractive surfaces such that in order from the second plane thefirst refractive surface of the second optical element group is concavetowards the second plane, and the second refractive surface of the thirdoptical element group is convex towards the first plane, the objectivecomprising an aperture stop, which is located at the second plane orbetween the second plane and the first optical element group, andwherein the objective is adapted to collect an image radius of R_(img)from the first plane, wherein an edge field marginal ray height from thethird optical element group (M_(rhG3)) satisfies the condition ofR _(img) −BFL*0.44<M _(rhG3) <R _(img) +BFL*0.44, wherein BFL is theback focal length (BFL) between the third optical element group and thefirst plane, measured in air.
 2. The objective according to claim 1,wherein the first refractive surface of the first optical element groupis convex towards the second plane.
 3. The objective according to claim1, wherein the first refractive surface of the first optical elementgroup is concave towards the second plane.
 4. The objective according toclaim 1, wherein the second optical element group comprises or consistsof a negative lens doublet, such as a negative meniscus-biconvex lensdoublet.
 5. The objective according to claim 1, comprising a prism orbeam splitter between the third optical element group and the firstplane.
 6. The objective according to claim 1, wherein the effectivefocal length of the second optical element group (f₂) is less than theeffective total focal length of the objective (f).
 7. The objectiveaccording to claim 1, wherein the absolute value of the ratio of theeffective focal length of the second optical group (f₂) to the effectivefocal length of the third optical element group (f₃) is between 0.3 and1.5.
 8. The objective according to claim 1, wherein the ratio of theback focal length of the objective (BFL) between the third opticalelement group and the first plane, measured in air, to the effectivetotal focal length of the objective (f) is between 0.4 and 1.5.
 9. Theobjective according to claim 1, wherein the third optical element groupcomprises two or more lens elements, the first of which having saidsecond refractive surface that is convex towards the first plane and atleast one of the other lens elements is a positive lens element.
 10. Awaveguide display device comprising: an optical waveguide, anin-coupling grating arranged at the waveguide so as to diffract lighthitting the in-coupling grating to the waveguide, and a projectionobjective arranged to project an image from an image source to thein-coupling grating, wherein the projection objective is an objectiveaccording to claim 1 and the optical waveguide comprises diffractivemeans for expanding a projection exit pupil along two dimensions. 11.The waveguide display device according to claim 10, wherein thein-coupling grating is arranged at the second plane of the projectionobjective.
 12. The waveguide display device according to claim 10,further comprising a means for presenting an image on the first plane ofthe projection objective.
 13. The waveguide display device according toclaim 10, wherein the waveguide further comprises an out-couplinggrating optically connected with the in-coupling grating for displayingthe projected image.
 14. The waveguide display device according to claim10, wherein the waveguide display device is a wearable personal displaydevice, in particular a head-mounted display (HMD) device, such as anear-to-eye display (NED) device.
 15. The objective according to claim1, comprising a prism or beam splitter as the first element of theobjective.
 16. The objective according to claim 1, wherein the absolutevalue of the ratio of the total focal length to the focal length of thesecond optical element group is less than 0.7.